1. Field of the Invention
The present invention relates generally to a video display apparatus and a method for controlling the apparatus. More particularly, this invention relates to a color video display apparatus implemented with at least one spatial light modulator to operate synchronously with a color changeover device applying a color sequential method.
2. Description of the Related Art
Even though there have been significant advances made in recent years in technologies of implementing electromechanical micromirror devices as spatial light modulators, there are still limitations and difficulties in providing high quality image displays. Specifically, when display images are digitally controlled, image quality is adversely affected due to an insufficient number of gray scales.
Electromechanical micromirror devices have drawn considerable interest because of their application as spatial light modulators (SLMs). A spatial light modulator requires an array of a relatively large number of micromirror devices. In general, the number of devices required ranges from 60,000 to several million for each SLM.
FIG. 1A shows a digital video system 1 that includes a display screen 2 disclosed in the relevant U.S. Pat. No. 5,214,420. A light source 10 is used to generate light energy for the ultimate illumination of display screen 2. Light 9 is further concentrated and directed toward lens 12 by mirror 11. Lens 12, 13, and 14 form a beam columnator to columnate light 9 into a column of light 8. A spatial light modulator 15 is controlled by a computer 19 through data transmitted over data cable 18 to selectively redirect a portion of the light from path 7 toward lens 5 for display on screen 2. FIG. 1B shows that SLM 15 has a surface 16 that includes an array of switchable reflective elements, e.g., micromirror devices 31, such as elements 17, 27, 37, and 47 as reflective elements attached to a hinge 30. When element 17 is in one position, a portion of the light from path 7 is redirected along path 6 to lens 5, where it is enlarged or spread along path 4 to impinge the display screen 2 so as to form an illuminated pixel 3. When element 17 is in another position, light is not redirected toward display screen 2 and, therefore, pixel 3 would be dark.
The on-and-off states of the micromirror control scheme as implemented in U.S. Pat. No. 5,214,420, and by most conventional display systems, impose a limitation on the quality of the display. Specifically, the conventional configuration of the control circuit has the limitations of a gray scale of conventional system (PWM between ON and OFF states) limited by the LSB (least significant bit, or the least pulse width). Due to the ON-OFF states implemented in conventional systems, there is no way to provide a shorter pulse width than the LSB. The minimum brightness, which determines gray scale, is the light reflected during the least pulse width. The limited gray scales lead to a degraded image display.
Specifically, FIG. 1C illustrates an example circuit diagram of a prior art control circuit for a micromirror according to U.S. Pat. No. 5,285,407. The control circuit includes memory cell 32. Various transistors are referred to as “M*” where “*” designates a transistor number, and each transistor is an insulated gate field effect transistor. Transistors M5 and M7 are p-channel transistors; transistors M6, M8, and M9 are n-channel transistors. The capacitances, C1 and C2, represent the capacitive loads presented to memory cell 32. Memory cell 32 includes an access switch transistor M9 and a latch 32a, which is the basis of the static random access switch memory (SRAM) design. All access transistors M9 in a row receive a DATA signal from a different bit-line. The particular memory cell 32 to be written is accessed by turning on the appropriate row select transistor M9, using the ROW signal functioning as a wordline. Latch 32a is formed from two cross-coupled inverters, M5/M6 and M7/M8, which permit two stable states. State 1 is Node A high and Node B low, and state 2 is Node A low and Node B high.
The dual states switching, as illustrated by the control circuit, controls the micromirrors to position either at an ON or an OFF angular orientation, as shown in FIG. 1A. The brightness, i.e., the gray scales of display for a digitally controlled image system, is determined by the length of time the micromirror stays at an ON position. The length of time a micromirror is controlled at an ON position is, in turn, controlled by a multiple bit word. For simplicity of illustration, FIG. 1D shows the “binary time intervals” controlled by a four-bit word. As shown in FIG. 1D, the time durations have relative values of 1, 2, 4, and 8 that, in turn, define the relative brightness for each of the four bits, where 1 is for the least significant bit and 8 is for the most significant bit. According to the control mechanism as shown, the minimum controllable differences between gray scales for showing different brightness is a brightness represented by the “least significant bit” that maintains the micromirror at an ON position.
When adjacent image pixels are shown with a great degree of different gray scales, due to a very coarse scale of controllable gray scale, artifacts are shown between these adjacent image pixels. That leads to a degraded image. The low quality of images is especially pronounced in bright areas of display when there are “bigger gaps” of gray scales between adjacent image pixels. It was observed in an image of a female model that there were artifacts shown on the forehead, the sides of the nose, and the upper arm. The artifacts are generated due to technical limitations produced by a digitally controlled display that does not provide sufficient gray scales. At the bright spots of display, e.g., the forehead, the sides of the nose, and the upper arm, the adjacent pixels are displayed with visible gaps of light intensities.
As the micromirrors are controlled to have a fully ON and fully OFF position, the light intensity is determined by the length of time the micromirror is at the fully ON position. In order to increase the number of gray scales of display, the speed of the micromirror must be increased so that the digital control signals can be increased to a higher number of bits. However, when the speed of the micromirrors is increased, a strong hinge is necessary for the micromirror to sustain the required number of operational cycles for the designated lifetime of operation. In order to drive the micromirrors supported on a further strengthened hinge, a higher voltage is required. The higher voltage may exceed twenty volts and may even be as high as thirty volts. The micromirrors manufactured by applying CMOS technologies probably would not be suitable for operation at this higher range of voltages, and, therefore, DMOS micromirror devices may be required. In order to achieve a higher degree of gray scale control, a more complicate manufacturing process and larger device areas are necessary when a DMOS micromirror is implemented. Conventional modes of micromirror control are therefore faced with a technical challenge since gray scale accuracy must be sacrificed for the benefits of a smaller and more cost effective micromirror display due to the operational voltage limitations.
There are many patents related to light intensity control. These patents include U.S. Pat. Nos. 5,589,852, 6,232,963, 6,592,227, 6,648,476, and 6,819,064. There are further patents and patent applications related to the different shapes of light sources. These patents include U.S. Pat. Nos. 5,442,414, 6,036,318 and Application 20030147052. The U.S. Pat. No. 6,746,123 discloses special polarized light sources for preventing light loss. However, these patents and patent application do not provide an effective solution to overcome the limitations caused by insufficient gray scales in digitally controlled image display systems.
Furthermore, there are many patents related to spatial light modulation, including U.S. Pat. Nos. 2,025,143, 2,682,010, 2,681,423, 4,087,810, 4,292,732, 4,405,209, 4,454,541, 4,592,628, 4,767,192, 4,842,396, 4,907,862, 5,214,420, 5,287,096, 5,506,597, and 5,489,952. However, these inventions have not addressed nor provided direct solutions for a person of ordinary skill in the art to overcome the above-discussed limitations and difficulties.
Therefore, a need still exists in the art of image display systems, which apply digital control of a micromirror array as a spatial light modulator, to provide new and improved systems that overcome the above-discussed difficulties.
Incidentally, in a so-called single-panel display system comprising one SLM, such as a digital micromirror device (DMD), as in the above described system, a color display is performed by converting the light, emitted from a white lamp light source, into a color sequential light by letting the light pass a rotating color wheel, and illuminating and modulation-controlling the SLM using the color sequential light, as disclosed in, for example, U.S. Pat. No. 5,371,543.
Furthermore, such a system, as disclosed in U.S. Pat. No. 5,448,314, for example, uses a color wheel and et cetera, which are shown in FIGS. 3A and 3B, together with the system shown in FIG. 2. The system shown in FIG. 2 is configured such that the RECEIVER 51 receives IMAGE INPUT, which is stored in MEMORY 52 and which is converted into an appropriate format by the PROCESSOR 53. Then the PROCESSOR 53 controls the LIGHT SOURCE 54, COLOR WHEEL 55, and DMD ARRAY 56 so that the light emitted from the LIGHT SOURCE 54 transmits itself in the COLOR WHEEL 55, which is reflected by the DMD ARRAY 56 and is projected onto the SCREEN 57. Specifically, the COLOR WHEEL 55 uses, for example, a color wheel that is segmented into three color filter sections, i.e., red (R), green (G), and blue (B) (these colors are sometimes collectively abbreviated as “RGB” hereafter), as shown in FIG. 3A, and a color wheel segmented into six color filter sections, i.e., RGBRGB, as shown in FIG. 3B.
In such a system, the spot of the light emitted from a light source (also noted as a “source light” hereafter) usually spans the border between the different color filters of the color wheel in rotation, causing the light transmitted through the color wheel to be a mix of colors during the period in which the light spans the border (the period is called a transition period or spoke period), resulting in degrading the color purity of the displayed video image.
For example, when the color wheel which is divided into three sections, i.e., RGB, is rotated, the light transmitted through the color wheel is a mixture of B and R during the period 62 in which the spot 61 of the source light spans the border between the B and R color filters, as shown in FIG. 4. Likewise, in the period spanning other borders, i.e., the border between R and G and between G and B, the light transmitted through the color wheel is a mixture of the colors of the color filters adjacent to each other on each border.
Accordingly, in to prevent degrading color purity due to such color mixtures, U.S. Pat. No. 6,972,777, for example, and the aforementioned U.S. Pat. No. 5,448,314 have proposed a method of blanking the display by means of an SLM (i.e., controlling the SLM under an OFF state) during a transition period.
The method, however, uses no light transmitted through the color wheel during the transition period, increasing periods in which the source light is not used and thus sacrificing the brightness of the display video image.
Specifically, the ratio of a transition period to a video display period of one frame is determined by the diameter of the spot of a source light transmitted through a color wheel and the number of divisions of individual color filters constituting the color wheel. For example, in a common color wheel divided into three sections of colors, i.e., R, G, and B, as shown in FIG. 3A, the ratio of the transition period is about 10%, while for a color wheel (i.e., a six-division color wheel comprising two sets of color filters, i.e., R, G, and B) divided into six sections of colors, i.e., R, G, B, R, G, and B, as shown in FIG. 3B, the ratio of the transition period actually exceeds 20%, resulting in a substantial loss in the volume of light (also noted as “light volume” hereafter).
Furthermore, related to the above described method is U.S. Pat. No. 5,592,188, for example, and others, which have proposed a method for improving the brightness of a display video image by controlling an SLM under an ON state during a transition period.
This method, however, is configured merely to equip the transition period of the video display period of one frame with the period of displaying white light, which degrades the contrast and color purity of the display video image.
As other methods U.S. Pat. Nos. 6,324,006, 6,445,505, et cetera, for example, have proposed a method of displaying a white video image during a transition period, while U.S. Pat. No. 6,567,134, for example, has proposed a method of displaying, during a transition period, the video image of a complementary color that is produced by mixing colors by means of two primary-color filters placed adjacent to each other.
These methods, however, need to generate white-color data and complementary-color data, respectively, to be used for display on the basis of the primary-color video signals, causing the circuit used for processing the video signal to become complex. Meanwhile, the video images of such white and complementary colors used only for the transition period are not capable of providing a sufficient level of gray scale representation or increasing the display period of a primary color, and, therefore, such a scheme does not contribute to improving the level of gray scale representation.
As yet another method, U.S. Pat. No. 6,972,777, and others, have proposed a method of using a color wheel placed between filters of the colors R, G, and B, with the color wheel sandwiching a white-color filter, which converts the transmitting light into white, or sandwiching a transparent filter.
This method, however, allows the light transmitted through the color wheel to be a mix of the primary color and white (W) during a period in which the spot of the source light spans the border between the primary-color filter and the white-color filter, which degrades color purity.
In FIG. 5, for example, the spot 66 of the source light is a mix of B and W during the period 68, which spans the border between the B-color filter and W-color filter 67. Spot 66 is likewise a mix of W and R during the period 69, which spans the border between the W-color filter 67 and R-color filter, which degrades color purity. A similar phenomenon occurs in other periods spanning the borders between the respective primary-color filters and W-color filters. Note that the transmission light is only white during a period in which the spot of the source light transmits itself through only the W-color filter (e.g., the period 70) instead of spanning the border between the primary-color filter and W-color filter.